1. Field of the Invention
This invention relates to an optical information processing apparatus and an optical pickup head used for recording, erasing or reprorducing information in the optical information processing apparatus.
2. Description of the Prior Art
A recent optical pickup head, with its optical system being simplified using a hologram, is shown in FIG. 1. (Refer to JP-A-62-188032, for example.) In FIG. 1, indicated by 2 is a semiconductor laser source. A laser beam 3 produced by the light source passes through a hologram 8 and is projected onto an objective lens 110, and then it is focussed on an optical disc 4. The beam reflected by the optical disk returns along the same optical path and is incident on the hologram 8. A diffraction beam produced by the hologram is incident on a detector 7. The diffraction beam is designed to have astigmatic aberration so that its form varies due to a defocussed condition of the objective lens with respect to the optical disc, as shown in FIGS. 2A, 2B and 2C. In the figures, FIG. 2B indicates an in-focus state, and FIGS. 2A and 2C indicate out-of-focus states. Indicated by 9 is a diffraction beam for producing a focus error signal FE, which is calculated from the photodetector outputs through the following calculation. EQU FE=(S1+S4)-(S2+S3) (1)
A tracking error signal TE is obtained by utilizing a diffraction image (far field pattern (FFP)) formed by tracks or pit strings on the disc. In FIG. 2, when the y-axis is defined to be parallel to the tracks or pit strings on the disc, the tracking error is calculated from the photodetector outputs through the following calculation. EQU TE=(S1+S2)-(S3+S4) (2)
In such an optical system, however, when the objective lens is moved by the operation of a tracking servo system, the incident position of the optical beam on the hologram surface is shifted by several hundred microns as shown by the dashed line in FIG. 3 (the figure omits the illustration of the optical system from the light source to the optical disc and a part of the optical beam). For this reason, the diffraction beam is moved about by 10% of the beam diameter on the photodetector, as shown by the dashed line in FIG. 4. Assuming that the y-axis defined to be parallel to the tracks or pit strings on the disc in FIG. 2, the diffraction beam is caused to move in the x-axis direction by the tracking control. In this case, an offset appears in the TE signal, as will be seen from the equation (2). In order to suppress this offset within an allowable limit, the lens is allowed to move through .+-.200 .mu.m at the greatest. Actually, the objective lens moves through about .+-.500 .mu.m thereby causing a tracking deviation to occur and hence signal detection to become impossible. The TE signal can also be produced in a different manner that other diffraction gratings 1021 and 1022 are provided at a part of the hologram 102 as shown in FIG. 5, and diffraction beams from the gratings 1021 and 1022 are detected by tracking error signal detectors 72 and 73 provided in addition to the focus error signal detector 71 in the photodetector 7. The TE signal can be obtained from a difference in the quantity of detected light. Since the phase difference between the 0-order diffraction beam and the first order diffraction beam coming from the tracks among the FFP is always .pi./2 even in the occurrence of defocusing, by providing the diffraction beam generation regions 1021 and 1022 for tracking error signal detection in addition to the diffraction beam generation regions 1023, 1024 and 1025 for focus error signal detection at the portions of the hologram on which the optical beam is incident, the tracking servo can be stabilized against the defocusing. Even in this case, however, the movement of the optical beam, which is incident on the hologram, due to the movement of the objective lens 110 causes an offset to be produced in the TE signal in the same way as the preceding case.
Another problem caused by the movement of the diffraction beam on the detector is the deterioration of characteristics of the focus error detection, such as deteriorated sensitivity, which results in the occurrence of defocusing. This problem also occurs in optical systems using lenses and half mirrors that are adopted in the optical pickup heads currently in the market.
When the collimating lens is omitted thereby to further reduce the number of component parts of the optical pickup head, as shown in FIG. 6, the optical beam diameter on the hologram 81 becomes smaller proportionately as the hologram departs from the objective lens 110. Therefore, it becomes necessary to adjust the position of the hologram to the optical beam more accurately. For example, when the hologram and photodetector are disposed to be apart from each other about by 5 mm so that they may be contained in a single package, the positioning of the hologram and the photodetector should be made with accuracy within .+-.50 .mu.m in the direction perpendicular to the optical axis.